Recently, in applications such as label package doubling as a protection of a glass bottle or a plastic bottle etc. and a display of articles, cap sealing and accumulation package, there has been widely used, as a shrink label, a polyester-based heat-shrinkable film which is high in heat resistance, easy to incinerate, and excellent in solvent resistance. The use amount of the polyester-based heat-shrinkable film tends to increase accompanied by an increase of PET (polyethylene terephthalate) containers.
Heretofore, a heat-shrinkable polyester film has been widely utilized which shrinks greatly in the width direction. It is also known that the shrinkage rate in the longitudinal direction, which is a non-shrinking direction, is made to be below zero (so-called extend due to heating) so that satisfactory shrinkage finishing properties are achieved (Patent Document 1).
Although the heat-shrinkable polyester film in which the width direction is the main shrinking direction is subjected to drawing at a high ratio in the width direction to exhibit the shrinkage properties in the width direction, with respect to the longitudinal direction perpendicular to the main shrinking direction, there are many cases in which the film is only subjected to drawing at a low ratio and there is also a case in which the film is not subjected to drawing. The film subjected to drawing at a low ratio in the longitudinal direction and the film subjected to drawing only in the width direction have a drawback that the mechanical strength in the longitudinal direction is poor. Moreover, when the film is subjected to drawing in the longitudinal direction to improve the mechanical strength in the longitudinal direction, the mechanical strength in the longitudinal direction increases, but the shrinkage rate in the longitudinal direction also increases, which in turn deteriorates shrinkage finishing properties. There are heat-shrinkable films that solve these drawbacks and thermally shrink only in the width direction while drawing in two directions of the longitudinal direction and the width direction described in Patent Documents 2 and 3. By drawing them biaxially, attenuation of shrinkage stress is small and good shrink finish can be obtained.
However, in the claim of Patent Document 2, the shrinkage stress is 3 MPa to 20 MPa, but in Example, it is as high as 8.2 MPa to 18 MPa. In the claim of Patent Document 3, although the shrinkage stress is 7 MPa to 14 MPa, but also in Example, it is 8.1 MPa to 13.3 MPa. This is presumed to be because the drawing stress in the width direction was increased by biaxial drawing.
However, the films of Patent Documents 1 to 3 having the main shrinking direction as the width direction did not have any problem in the containers of the PET bottles for beverages, but there are packages having thin container thickness, such as box lunches and prepared dishes sold at convenience stores and supermarkets and such packages are used with thinner containers for achieving the reduction in the weight of garbage in recent years. When a heat shrinkable film having a highly shrinkage stress is used in a container having a small thickness, problems such as deformation of the container due to shrinkage stress of the film occur at the time of shrinking.
Moreover, in the case of preparing a label for a bottle for beverage from a heat-shrinkable film, it is necessary that the film be formed into a tube shape to be mounted to a bottle and then be allowed to shrink by heat in the peripheral direction of the bottle. For that reason, in the case of forming a heat-shrinkable film which is heat-shrinkable in the width direction into a label, while a tube body is formed such that the width direction of the film becomes the peripheral direction, this tube body must be cut into a label with a prescribed length to be mounted to a bottle. Accordingly, there has been a restriction on the speed for mounting a label composed of a heat-shrinkable film which is heat-shrinkable in the width direction to a bottle, and improvement thereof has been desired.
On that account, recently, a film which is capable of being wound around a bottle directly from a film roll to be formed into a label (the so-called wrap-around) and is heat-shrinkable in the longitudinal direction has been desired. Furthermore, in recent years, a wrapping method of covering the circumference of a synthetic resin-made container for a box lunch or the like with a belt-shaped film to hold the container in a closed state has been developed, and the film which is shrinkable in the longitudinal direction is suitable for such a packaging application. Accordingly, the demand for the film which is shrinkable in the longitudinal direction is expected to dramatically increase in the future.
However, in the film shrinking in the longitudinal direction (Patent Documents 4 to 5), the shrinkage stress in the claim of Patent Document 4 is 6 MPa or more and is as high as 7 MPa or more in Example. In Comparative Example 1, there are films having a shrinkage stress as low as 3.9 MPa, but this is considered due to a low shrinkage rate of 26%. The shrinkage stress in Patent Document 5 is also 6 MPa or more which is similar to that in Patent Document 4, and is as high as 7 MPa or more in Example. In Comparative Example 5, some films have a low shrinkage stress of 4.7 MPa, but this is considered due to a low shrinkage rate of 33%. Therefore, a film requiring a high shrinkage rate and a low shrinkage stress was not obtained. This is presumed to be because the drawing stress in the longitudinal direction was increased by biaxial drawing. However, there was no problem in containers of PET bottles for beverages, but packages with a thin container thickness, such as boxed lunches and side dish containers sold at convenience stores and supermarkets, have recently become thinner for achieving the reduction in the weight of garbage, and thin containers are used therefor in some cases. When a heat shrinkable film having a highly shrinking stress is used in a container having a small thickness, problems such as deformation of the container due to shrinkage stress of the film occur at the time of shrinking.